1. Field of the Invention
The present invention relates to granules of vinyl acetate/ethylene copolymer coated with a polar thermoplastic polymer, to a process for producing such granules and to the use of these granules as adhesives. Vinyl acetate/ethylene copolymer is understood to mean vinyl acetate/ethylene copolymers having a preponderant content of vinyl acetate.
2. Description of the Related Art
It is well known to combine, by coextrusion, within multilayer structures, bulk polymers such as, for example, polyvinyl chloride or polyolefins with so-called special-purpose polymers imparting special properties such as resistance to corrosion or to ultraviolet rays or alternatively impermeability to gases, odours and/or water vapour.
Vinyl acetate/ethylene copolymers as defined above constitute effective coextrusion adhesives, used alone or mixed with other adhesive polymers, to cause bulk polymers such as vinyl chloride polymers or polyolefins to adhere to special-purpose polymers such as vinylidene chloride copolymers, vinylidene fluoride polymers, vinyl alcohol/ethylene copolymers (EVOH copolymers), polyamides, polyethylene terephthalates ("PET"), and the like. The use of coextrusion adhesives based on vinyl acetate/ethylene copolymers is described in Patent Applications EP-A-0,124,931 of 17 Apr. 1984, EP-A-0,164,766 of 10 Apr. 1985, EP-A-0,259,899 of 30 Jun. 1987 and EP-A-0,323,672 of 27 Dec. 1988, all in the name of SOLVAY & Cie.
Vinyl acetate/ethylene copolymers, hereinafter designated by the term "VAE copolymers", nevertheless possess the well-known drawback of being highly tacky and of usually taking the form of an agglomerated mass, making their use arduous. To remedy this drawback, it has already been recommended to coat vinyl acetate/ethylene copolymers by means of a hard thermoplastic polymer such as, for example, polyvinyl chloride (see, for example, Patent BE-A-697,025 of 14th Apr. 1967 in the name of SOLVAY & Cie). According to this document, the coating is carried out by coagulation of a latex of a hard polymer, such as polyvinyl chloride, on a coagulated latex of VAE copolymer. The coated VAE copolymer, isolated by draining followed by drying, takes the form of a dry powder of poorflowability. In addition, in the case where the coextrusion adhesive consists of a mixture of VAE copolymer with another adhesive polymer, its preparation will also involve a subsequent operation of mixing and of granulation of the mixture of adhesive polymers.
The objective of the present invention is to provide vinyl acetate/ethylene copolymers coated with a thermoplastic polymer which are usable as coextrusion adhesives not possessing the abovementioned drawbacks.